JPH065267B2 - Josephson junction high-sensitivity magnetometer - Google Patents

Description

TECHNICAL FIELD The present invention relates to a high-sensitivity magnetometer having a Josephson junction.

(Prior Art) SQUID (superco
nducting quantum interference device) is characterized by high sensitivity and extremely fast response, and is used in fields requiring high-accuracy measurement and high-sensitivity measurement. For example, the detection of a weak magnetic field signal generated by the human body is SQUID
Made possible by the magnetometer. In addition, it has become possible to accurately perform geological surveys such as the existence of oil field layers and exploration of high temperature water layers.

As SQUID, RF as shown in Fig. 2 is used today.
-SQUID and DC-SQUI as shown in FIG.
D is known.

The DC-SQUID can optimize the operation bias voltage and has high sensitivity as compared with the RF-SQUID, but has a drawback that it is easily damaged by a charged electric charge of a human body, etc. Only SQUIDs are in practical use. In the RF-SQUID shown in FIG. 2, the self-inductance of each superconducting loop 50, 51 is L,
The currents flowing through the superconducting loops 50 and 51 are I ₁ and I _2, and the external magnetic fluxes given to the superconducting loops 50 and 51 are Φ _e1 and Φ _e2 , respectively.
The internal magnetic fluxes of 0 and 51 are Φ ₁ and Φ ₂ , respectively,
Further the critical current value of the Josephson junctions 52 and I _m, the equation representing the RF-SQUID of the operation shown in FIG. 2 Given in. here Represents a unit magnetic flux quantum.

FIG. 5 shows the relationship of the internal magnetic flux Φ _{1 to} the excited external magnetic flux Φ _e = (Φ _e1 −Φ _e2 ) / 2 in the RF-SQUID shown in FIG. 2. Curves A and B have different critical current values I _M
However, according to the theory of noise, in the case of the curve B in FIG. 5, the ratio of the input magnetic flux to the output magnetic flux at the operating point is the maximum, and the maximum sensitivity can be obtained. Curve B is realized when Φ ₀ = 2πLI _M (2).

An example of the RF-SQUID magnetometer shown in FIG. 2 is described in Japanese Patent Application No. 57-225633. The RF-SQUID described in this specification is formed by a thin film laminating technique in which an insulator thin film and a superconductor thin film are laminated, and the Josephson junction is achieved by a weak bond. This thin film type RF-SQUID element has the possibility of mass production as compared with the conventional point contact type.

(Problems to be solved by the invention) As described above, the critical current value of the Josephson junction is the second (2)
It is a condition to obtain the maximum sensitivity that it is set to satisfy the formula, but it is the second (2) in the conventional RF-SQUID.
It is extremely difficult to satisfy the condition I _M = Φ ₀ / (2πL) of the formula.

For example, in the thin-film type SQUID as described in the above-mentioned Japanese Patent Application No. SQID, it is not easy to calculate or measure the inductance of a small superconducting loop formed by a thin film, and it is possible to accurately find a value to be set. There is also the problem that it cannot be done, but a critical current value I of about 10 μA
There is the problem that it is extremely difficult to adjust _M accurately.

When weak coupling is used as the Josephson junction, the dimensions of the weak coupling portion are first determined to be, for example, 0.5 μm, a thickness of 150 Å, and a length of 100 Å at the design stage in order to obtain a desired critical current value. It is usual to adjust the dimensions of the weak connection at the manufacturing stage,
Actually, the critical current value is considerably less than the planned value determined by Eq. (2).

Josephson case of using weak bonds as bonding (not be said for other Josephson junction) by anodic oxidation after the production of an exceptionally Once the Josephson junction can be adjusted critical current value I _M, the In some cases,
Since the critical current value cannot be measured at the same time during the anodization, it is necessary to alternately perform the anodization and the critical current value measurement.
In many cases, it does not reach the planned value even if repeated. On the contrary, in some cases, the critical current value becomes too smaller than the expected value and the operation fails. The critical current value I
It is only possible to make _M small.

Therefore, conventionally, the liquid helium temperature was changed during the operation of the magnetometer to change the critical current value I _M (the critical current value has temperature dependency) to satisfy the condition (2) of the highest sensitivity. However, from a practical point of view, changing the liquid helium temperature is extremely troublesome, and the cost therefor is not preferable.

An object of the present invention is to solve the above problems. That is, to provide a second (2) the critical current value I _m which satisfies can be easily obtained, Josephson junction sensitive magnetometers to measure the maximum sensitivity can be easily realized.

(Means for Solving the Problems) As shown in FIG. 1, the Josephson junction high-sensitivity magnetometer of the present invention includes a superconducting control loop 3 having two Josephson junctions 1 and 2, and one The superconducting control loop 3 is composed of two superconducting input / output loops 4 and 5 which share the Josephson junction 1 or 2, and a control line 6 magnetically coupled to the superconducting control loop 3 is arranged in close proximity thereto. Further, at least one of the superconducting input / output loops 4 and 5 (both in the figure), an input / output line 7 which is also magnetically coupled to the superconducting input / output loops 4 is arranged in proximity. Here, the magnetic transformer coupling means that the superconducting control loop 3 and the control line 6 and the superconducting input / output loops 4 and 5 and the input / output line 7 are magnetically coupled via a magnetic field.

The magnetometer of the present invention is considered to have an intermediate structure between the RF-SQUID shown in FIG. 2 and the DC-SQUID shown in FIG. This means that the magnetometer of the present invention uses the RF-SQUID Josephson coupling as the DC-SQ.
It is also apparent from the fact that the circuit shown in FIG. 4 substituted with the UID can be obtained by the electric network conversion such as the II-Δ network conversion law. Therefore, the magnetometer of the present invention is called a hybrid SQUID. You can

The hybrid SQUID of the present invention has the advantages of RF-SQUID and DC-SQUID, has high sensitivity, and has no problem of damage due to electric charge.

(Operation) The characteristics of the hybrid SQUID of the present invention are described by the following equations.

Ψ ₁ is as defined in FIG. 1b, Ψ ₁ = Φ ₁
A + (Φ _3/2). Further, Φ _e3 and Φ ₃ are the external magnetic field and the internal magnetic field of the control loop 3, respectively. FIG. 6 shows the relationship of the internal magnetic flux Ψ ₁ to the excited external magnetic flux Φ _e = (Φ _e1 −Φ _e2 ) / 2. In the present invention, it is understood that the relationship between Ψ ₁ and Φ _e changes variously only by changing the external magnetic flux Φ _e3 for control.

Comparing equation (1) with equation (3) It can be seen that

This is because in the DC-SQUID of FIG. 3, I = I ₁ + I ₂ = I _m sin (θ + δ) + I _m sin (θ−δ) = 2I _m cos δ · sin θ = I _M sin θ (5) holds. As you can see, equation (4) is a typical D
This is the same as the modulation of the critical current value by the magnetic field in C-SQUID.

The hybrid SQUID of the present invention has a shape similar to that of the RF-SQUID, but also functions as a DC-SQUID. The operating characteristics of the present invention are RF-SQUID and D
The fact that it is intermediate between C-SQUID is actually
It has already been proved in equation (3).

As shown in FIG. 7, the variable resistor 53 is used in the present invention.
By changing Φ _e3 , the effective critical current of the Josephson junction can be changed according to the equation (3), and various characteristics as shown in FIG. 6 can be obtained. In order to change the characteristics of the conventional RF-SQUID shown in FIG. 2, it is necessary to use a troublesome and expensive means of changing the temperature of liquid helium.

Since the present invention satisfies the expression (4), the condition of the expression (2) can be rewritten as follows.

This is very different from the case of the conventional RF-SQUID. That is, in manufacturing, when the weak coupling is used as the Josephson junction and lithography, the dry etching technique, the critical current I _m is made to be an appropriate value satisfying I _m Φ ₀ / (4πL) by the anodic oxidation method. deep. It need not be exactly I _m = Φ ₀ / (4πL). During operation, Φ _e3 can be adjusted to satisfy equation (6) and obtain maximum sensitivity. 7th to change Φ _e3
It is only necessary to change the variable resistor 53 in the figure, and the conventional RF
The conditional expression (6) with the highest sensitivity can be satisfied without changing the liquid helium temperature as in the case of -SQUID.

(Example) Hereinafter, the Example of this invention is described. FIG. 8 is a plan view showing the actual configuration of the magnetometer shown in FIG.

A superconducting control loop 3 having two Josephson junctions 1 and 2 is provided with a first superconducting thin film 11a provided on an insulating substrate such as SiO ₂ and a first superconducting thin film 11a provided on the thin film 11a via an insulating film. The second superconducting thin film 12a and the third superconducting thin film 13
It is formed by weakly coupling with a and 13a '. The two superconducting input / output loops 4 and 5 include a first superconducting thin film 11b and 11c and a second superconducting thin film 12b and 12c.
And are coupled by the third superconducting thin films 13b and 13c. The weak bond forming the Josephson junction is shared with the weak junction of the superconducting control loop 3. The control line 6 for applying the magnetic flux to the control loop 3 is formed by the second superconducting thin film 12d, and the input / output loop for inputting / outputting the magnetic flux to / from the input / output loops 4 and 5 is excluding the intersecting portion. The first superconducting thin film 1
It is formed by 1d. In this example,
Since the configuration of the magnetometer is bilaterally symmetric and the input / output signals are bilaterally antisymmetric, it is unlikely to be affected by an external magnetic field such as geomagnetism.
In this magnetometer, four masks are required to manufacture the magnetometer, which is exactly the same as that of the conventional magnetometer without a control loop. Is never complicated. The superconducting thin film can be made of Nb or the like, and the insulating film can be made by oxidizing the superconducting thin film.

FIG. 9 shows the control loop 3 and the input / output loops 4 and 5 connected to the control line 6.
FIG. 6 is a plan view of an embodiment in which the input / output line 7 and the input / output line 7 are formed separately from each other. Reference numerals in the figure are the same as those in FIG.

FIG. 10 is a schematic view of a magnetic flux measuring device using the magnetometer of the present invention.

The terminal 32 of the input / output line 31 and the capacitance 3 of the magnetometer 30 of the present invention
3 are connected in parallel, and the parallel resonance circuit including the input / output line 31 and the capacitor 33 is connected to the rf oscillator 34, the rf detector 35, and the af oscillator 42. When SW ₁ is open, a direct current is applied to the control line 36 of the magnetometer 30 by the direct current power source 37.
The value of the current flowing through the control line 36 is adjusted by the variable resistor 38, and the effective critical current value _IM is set to the optimum value.
One end of the magnetic flux measurement pickup coil 39 is formed in a loop shape in accordance with the input / output loop 40 of the magnetic flux meter 30, and is installed on the input / output loop 40.

When the af oscillator 42 and the rf oscillator 34 apply the af modulation magnetic flux to the input / output loop 40 via the input / output line, the phase detector 41 obtains a value obtained by differentiating the rf signal by the measured magnetic flux. That is, the conventional control measurement circuit for RF-SQUID can be used as it is.

SW ₁ is closed, SW ₂ is switched to the b side, and control loop 4
The high frequency signal from the rf oscillator 43 is applied to 4 and the signal of the rf oscillator 43 is used as the reference input of the phase detector.
If the difference frequency signals f ₁ -f ₂ from the signal of the rf oscillator 34 are used, the measurement can be performed in exactly the same manner as described above.

Hereinafter, another embodiment of the present invention will be described. In the embodiment shown in FIG. 11, the first superconducting input / output loop 4 is closed by a circuit a, b, c, a, and the second superconducting input / output loop 5 is closed by a circuit d, b, c, d. Are formed and the lines b and c are shared by the first and second superconducting input / output loops 4 and 5.

In the embodiment shown in FIG. 12, the first superconducting input / output loop 4 is closed by the closed circuits a, b, c, a, and the second superconducting input / output loop 5 is closed by the closed circuits d, b, c, d. The conduction control loop 3 is formed by closed circuits a, b, c, d, a.
This embodiment is obtained by converting the embodiment of FIG. 11 into a T-bridge circuit. Further, when the bridge-π circuit class conversion of this embodiment is performed, the configuration shown in FIG. 1 can be obtained.

The embodiment of FIG. 13 is one in which the Josephson junctions 1 and 2 are installed on the outer periphery of the circuit, and the quantization conditions of the internal magnetic flux are completely different from the above embodiments. The first superconducting input / output loop 4 is formed by closed circuits a, b, c, a, and the superconducting control loop 3 is formed by closed circuits a, c, d, a.

In the embodiment of FIG. 14, the first superconducting input / output loop 4 is formed of closed circuits a, d, c, a, and the second superconducting input / output loop 5 is closed circuit b, c, d ,. b, and the superconducting control loop 3 has the closed paths a, d, c, b,
It is formed from a. This embodiment can be obtained by converting the embodiment of FIG. 13 into a T-bridge circuit.

In the embodiment of FIG. 15, the first superconducting input / output loop 4 is formed of closed circuits a, e, f, c, a, and the second superconducting input / output loop 5 is closed circuit b, d ,. The superconducting control loop 3 is formed of f, e, b,
e, a. This embodiment can be obtained by converting the embodiment of FIG. 14 into a bridge-Y circuit.

In each of the above embodiments, the input / output line 7 is arranged close to the first and second superconducting input / output loops 4 and 5, but the first and second superconducting input / output loops 4 and 5 are arranged. The magnetic flux can be measured even if the input / output line 7 is arranged close to only one side.

Further, as the Josephson junction, any conventionally known one can be used.

(Effect of the Invention) As described in detail, in the SQUID high-sensitivity magnetometer of the present invention, the peak of sensitivity can be easily realized only by adjusting the bias magnetic field applied to the superconducting control loop during operation. Because of this, it has the following advantages over the conventional RF-SQUID.

(1) The strictness on the adjustment of the critical current of the Josephson junction by the anodic oxidation method was greatly eased, the production yield was improved, and the original mass production of thin films became possible.

(2) Even if the critical current of Josephson manufacturing changes over time, it does not mean performance deterioration or damage as in the past. Adjustment control loop The condition that gives the peak of sensitivity can be completely satisfied only by shifting the bias magnetic flux applied, and the maximum sensitivity is reproduced. This means that the life of the device is extended and that the device can always be used with maximum sensitivity.

(3) To operate at the point of maximum sensitivity, it is only necessary to change the variable resistance of the control power supply, and it is no longer necessary to perform the difficult and expensive operation of changing the temperature of liquid helium as in the past. .

(4) Josephson junctions whose critical current values other than weak coupling cannot be changed after manufacturing can also be used.

[Brief description of drawings]

1a and 1b are conceptual diagrams of an embodiment of the present invention, FIG. 2 is a diagram showing a conventional RF-SQUID, FIG. 3 is a diagram showing a conventional DC-SQUID, and FIG. 4 is an RF-SQUID. DC of Josephson junction of
FIG. 5 is a graph showing a characteristic of RF-SQUID, FIG. 6 is a graph showing a characteristic of hybrid SQUID of the present invention, and FIG. 7 is a control means of hybrid SQUID of the present invention. Fig. 8, Fig. 8 and Fig. 9 are plan views showing an example of the actual structure of the magnetometer of the present invention, Fig. 10 is a schematic view of the magnetic flux measuring device used in the magnetometer of the present invention, and Figs. FIG. 15 is a conceptual diagram of another embodiment of the present invention. 1, 2 ... Josephson junction 3 ... Superconducting control loop 4 ... First superconducting input / output loop 5 ... Second superconducting input / output loop 6 ... Control line 7 ... Input / output line

Claims (1)

[Claims] 1. A superconducting control loop having two Josephson junctions, a first superconducting input / output loop that shares one of the Josephson junctions with the control loop, and the other Josephson junction with the control loop. A Josephson junction high-sensitivity magnetometer comprising a shared second superconducting input / output loop and a control line disposed in proximity to the superconducting control loop and magnetically coupled to the superconducting control loop.